In a spread band mobile communication system of a code division multiple access type (To be called "CDMA" below), one of the major functions of a mobile station is to form synchronization between spreading communication signals and the spread band signals, so that the signals can be decoded to the original form before the spreading.
In the CDMA mobile communication system, pilot pseudo random sequence signals having a length of 2.sup.15 (32,768) chips are used, so that the mobile station can achieve a synchronization with spread band signals sent from a base station.
That is, the pilot pseudorandom sequence signals are used as spreading signals for spreading the band width of the digital signals.
The pseudorandom sequence signals are provided not only as a phase reference for matching the synchronization, but they are used as a means for comparing the signal intensities between base stations.
In order to discriminate the forward CDMA channels which use the same codes, the base stations use a time offset of the pilot pseudorandom sequence.
That is, the base station have mutually different offset values.
Therefore, in the CDMA mobile communication system, if a mobile station is to achieve a synchronization with the signals of base stations, and during a passing through a boundary of cells, if a mobile station is to be quickly adapted to the signals of a base station having a different offset value so as to receive the signals, first of all, there is required a pseudorandom noise be called "PN" below) generator in which the mobile station can easily move the PN.
In other words, generally in the direct sequence spreading band communication, the PN sequence generator is used to match the received and transmitted signals. During the receiving and transmitting of signals in the CDMA mobile communication system, signals are encoded or decoded into spread spectrum by utilizing the PN sequence. That is, PN sequences having different offset values are assigned to the respective base stations, so that the PN sequences of the respective base stations can be distinguished. Therefore, the respective base stations can re-use the frequencies, and therefore, many subscribers can be accommodated within the permitted frequency band.
In the direct sequence spreading band communication system such as CDMA, the PN sequence is formed by a linear sequence shifter register (to be called LSSR).
The LSSR, i.e., the PN sequence generator includes: an N-stage shift register; and a plurality of exclusive OR gate added to the shifter register for programming the PN sequence.
The locations of the exclusive OR gates are determined by a defining polynomial of circuit which determines which sequence among the possible sequences has occurred.
For the N-stage generator, a total of 2.sup.(N-1)- 1 polynomials exist. Among them, only a part (about 10%) generates the maximum length.
For example, a PN sequence generator which has 15 stages and the maximum polynomials generates a sequence of 32,762 bits or chips.
The pilot PN sequence which is normally used in the CDMA spread band mobile system is formed from the following polynomials in accordance with the phase.
&lt;In-phase&gt; EQU P.sub.I (X)=1+X.sup.5 +X.sup.7 +X.sup.8 +X.sup.9 +X.sup.13 +X.sup.15( 1)
&lt;Quadrupture-phase&gt; EQU P.sub.Q (X)=1+X.sup.3 +X.sup.4 +X.sup.5 +X.sup.6 +X.sup.10 +X.sup.11 +X.sup.12 +X.sup.15 ( 2)
Of the above polynomials, the PN sequence generator which corresponds to Formula 1 consists of 15 stages as shown in FIG. 3.
The length of the PN sequence which is generated by the 15-stage LSSR is 2.sup.15 (32,767) chips or bits.
In actually constituting the PN sequence generator, generally the length of a 2.sup.N -1 sequence, i.e., most of the numbers (1, 3, 5, 7, 15, 31, . . . , 255, 511, . . . , 32767, . . . 2.sup.N -1) are prime numbers having no factors. Therefore, their applications are not suitable.
This makes it difficult to synchronize the systems which are operated with a low rate compared with the PN chip rate.
In an actual example, for a data modulation rate of 9600 bits per second, a PN sequence rate of 1.2288 MHz is required.
The information bits are subjected to an exclusive Or with the PN sequences, and the result is subjected to a 2-phase modulation with an RF carrier for transmissions.
Thus a PN of 128 chips per information bit is provided.
In other operating modes, the PN rate is same, but the data rate is reduced to 4800 per second (256 chips per information bit).
That is, when a PN sequence which has a length of 2.sup.N-1 is to be applied to the actual spreading band system, particularly when various data rates are used, it is difficult to synchronize the signals.
It is desirable that the data modulations for the repetition of the PN sequences are synchronized. However, if the PN sequence has a length of 32767 (i.e., 2.sup.15 -1) having 7, 3, and 151 as the minor factor, then the repetition gap of the PN code and the above described two data rates correspond with each other only in every 128 or 256 repetition gap.
Such correspondence is formed every 3.4 seconds, or every 6.8 seconds, and no correspondence is seen in other gaps.
In view of this, it is desirable that the length of PN sequence is 2.sup.N in which the correspondence frequency is increased in the repetition gaps of the PN sequence for the multiple data rates.
For this purpose Qualcomm company of the United States describes in its U.S. Pat. No. 5,228,054 a PN sequence generator which generates a PN sequence having a length of 2.sup.N for using it in the CDMA spread band mobile communication system. The constitution of this PN generator is illustrated in detail in FIG. 14.
Referring to FIG. 14, this conventional PN sequence generator roughly includes: an LSSR 10 for generating a PN sequence having a length of 2.sup.N -1; a circuit lengthening circuit composed of an N-bit comparator 20, two D flip-flops 21 and 22, an invertor 24, a NAND gate 26, and an AND gate 28, for inserting an additional bit to a proper bit position within the PN sequence output of the LSSR 10; and a random sequence moving circuit composed of a mask circuit 30, D flip flops 40 and 58, a multiplexer 42, a binary counter 48, a comparator 52, and logic gates 44, 46, 50, 54 and 56.
In this technique, 14 zeros `0` continuously occurs within the PN sequence (this occurs only once per sequence cycle), and then, one `0` is added to form a PN sequence having a length of 2.sup.15 (=32,768), so that it can be applied to the system.
In this case, if various data such as 1200, 2400, 4800, 9600 bps and the like are applied, synchronization are easily formed between the data and the spreading signals.
However, in this conventional apparatus, the input of the comparator 20 which is for inserting an additional bit into the PN sequence output is fixed to `0 . . . 0100`. Therefore, a binary counter 42, an offset comparator 52 and other logic gates are required, with the result that the constitution of the circuit is complicated.
Further, there should be prepared in advance a PN mask lookup table which occupies a large amount of memory, and therefore, the efficiency of the system operation is lowered.